1. Field of the Invention
This invention relates to the field of drying thin coatings on materials, and more particularly to a method and means for drying a thin aqueous-based coating on heat sensitive materials without boiling and without distortion or damage to the heat sensitive material or delicate surface thereof.
2. Brief Description of the Prior Art
In the process of drying thin coatings deposited on articles of manufacture, several problems are encountered. In particular, the coating to be applied to an article is generally selected for certain important characteristics, such as being inert to the material upon which it is being deposited, making a good bond with the material, the absence of hazardous vapor produced in the evaporation of the liquid base of the coating, and immunity to distortion or deformation of the article or the surface thereof during the drying process. In a high production process, the drying of "wet" coatings is accellerated by the provision of heating devices to raise the temperature of the coating and therefore reduce the time necessary for drying the coating. Although such problems as just enumerated, and the solutions to such problems, are applicable to a variety of coated articles of manufacture, the background information and description portion of this application will be concerned primarily with the drying of coatings on flat surfaces, although the principles of the invention may be applied to the drying of coatings on any type of material, especially those materials sensitive to heat.
In the production of videodiscs, typically two disc halves (each disc half referred to as a 1X disc) are bonded together by an adhesive. The two sided (2X) discs are assembled by applying a light spray of solventbased adhesive onto one or both inner surfaces of the 1X discs, and the two 1X discs are then assembled by using an assembly plate with a center hole aligning guide. The assembled discs are then pressed together using either a platen press or a rotary press. After the discs are so assembled, they are routed to an edging and cleaning station for smoothing the periphery of the disc and cleaning the playing surfaces.
An example of prior art method and means for bonding two 1X videodiscs together can be found by a reference to U.S. Pat. No. 4,313,190, issued Jan. 26, 1982 to Gary G. Slaten and assigned to the assignee of the present invention. In that U.S. Patent, the bonding material, typical in the art, is Pliobond manufactured by the Goodyear Company. Pliobond is a stable colloidal dispersion of Neoprene Type AH in an aliphatic solvent such as heptane.
Solvent-based adhesives are used in the production of videodiscs because of the ability to assemble the 1X discs immediately after the spray adhesive is applied. However, the consequences of using a solvent-based adhesive are many. Probably the most important ones lie in the fact that certain solvent-based adhesives attack the surface of the item to be covered, and in the case of videodiscs, the solvent in the adhesive may well contaminate the plastic surfaces or even the aluminum reflective layer of a laser disc. Such attacks on the information bearing surfaces of a videodisc can, of course, be catastrophic, necessitating the choosing of a bonding adhesive which will provide a strong bond and yet not interact chemically or physically with the surface of the article being coated. Other disadvantages of using solvent-based adhesives, are, relative to other coatings such as aqueous-based coatings, high cost, hazardous and unhealthy vapor products, the nuisance of excessive waste on parts of the disc handling apparatus during and subsequent to applying the adhesive, and time consuming and expensive clean up operations.
An alternate to solvent-based adhesives is, of course, water based adhesives, but this choice would be immediately dismissed by one skilled in the art due to the excessive drying time needed, and this disadvantage would be particularly prohibitive in high production environments.
Accordingly, there is a need in the art for developing a coating application and drying process in which the above-enumerated disadvantages of using solvent-based coatings are avoided and the excessive drying time for water-based coatings is eliminated. Water-based adhesives are used in woodworking (laminating) processes where the production rate is small, where the substrate is not heat sensitive, and where boiling may not be a problem. In such a case, the adhesive may merely be allowed to dry for several hours at room temperature, an infrared lamp may be used to accelerate the drying of the coating, or the coated substrate can be inserted into a hot air oven or otherwise exposed to a mass of hot moving air. However, it has heretofore been unknown how to use and gain the benefits of a water based adhesive in regard to the production of videodiscs and yet avoid the extensive drying times prohibitive for high volume production. The present invention satisfies this need.